Papermaking involves the forming, pressing and drying of cellulosic fiber sheets. The forming process includes the step of depositing an aqueous stock solution of the fibers, and possibly other additives, onto the forming fabric upon which the initial paper web is formed. The forming fabric may run on a so-called Gap Former machine in which the aqueous stock initially is dewatered, and the initial paper sheet is formed between two forming fabrics.
An effective forming process typically produces a sheet with a very regular distribution of fibers and with a relatively high solids content, i.e., a high fiber-to-water weight ratio. In order to form a fibrous web with a desired uniform, regular distribution and high fiber-to-water weight ratio, the forming fabric must possess a number of properties. First, the papermaking surface should be relatively planar; resulting from the yarn floats in both the machine direction (MD) and cross-machine-direction (CD) lying at substantially the same height, to thereby prevent localized penetration of the fibers into the fabric. Such localized penetration results in “wire marks” which actually is the result of fiber basis weight variations throughout the sheet area. In addition, the MD and CD floats need to be distributed in a regular manner to avoid introducing undesired wire marks into the formed sheet. Moreover, these basis weight variations can result in undesired variations in sheet absorption properties; a property very relevant to the functionality of quality graphical papers where a consistent uptake of print ink is necessary to produce a clear sharp image.
Other factors also cause the formation of undesired wire marks. For example, wire marks can be introduced into the sheet by the flow of water around yarns positioned below the fabric's papermaking surface. This phenomena, referred to as “strike through,” needs to be taken into account in designing the fabric construction.
Importantly, the forming fabric must also possess a high degree of dimensional stability. This high stability is necessary, for example, to minimize cyclic variations in fabric width, which can result in MD wrinkles in the fabric. This, in turn, contributes to the so-called, streaky sheet, i.e., a sheet with MD streaks created by variations in fiber basis weight.
Dimensional stability of a fabric typically is obtained by manufacturing the forming fabric with a relatively high mass of material. However, the use of thick yarns often causes undesirable wire marks. Consequently, there has been a trend to providing composite forming fabrics, that is, “multi-layer” structures, whereby a high number of relatively thin yarns are distributed throughout various fabrics layers to facilitate fabric stability.
One type of multi-layer structure is a triple-layer, or composite, fabric made by joining two (2) distinct fabrics, each with their own MD (warp) yarns and CD (weft) yarns, by the use of additional and independent “binding yarns.” These binding yarns can be employed in either the MD or CD direction, and in this system provide the sole function of binding the two separate fabrics together. In other words, these binding yarns are not intended to function as part of the warp or weft yarn system in either the top fabric or the bottom fabric of the multi-layer structure. Such a triple-layer fabric is illustrated in EP 0,269,070(JWI Ltd.).
Where the two fabrics of the triple-layer structure are joined in either the MD or CD direction by binding yarns that also belong, or form part of the weave pattern of either, or both, of the paper side or wear side fabrics, the resulting structures are referred to more specifically as “self-stitched” triple-layer structures. Such binding yarns are referred to as intrinsic binding yarns. Self-stitched structures are taught in a number of prior art patents. For example, U.S. Pat. No. 4,501,303 (Nordiskafilt AB) discloses a triple-layer structure wherein paper side yarns are used to bind the paper side and wear side fabrics into one structure.
Triple-layer structures, whether employing separate and distinct binding yarns or intrinsic binding yarns that form part of the paper side and/or wear side weave structure, allow, to some extent, for the use of fine MD and CD yarns in the paper side fabric layer for improved papermaking quality and sheet release. Additionally, the use of significantly coarser yarns can be employed in the lower fabric layer, or wear side fabric layer, which contacts the paper machine elements, to thereby provide good stability and fabric life. Thus, these triple-layer structures have the capability of providing optimum papermaking properties in the paper side fabric layer and optimum mechanical properties in the wear side fabric layer.
In the known commercial embodiments of the triple-layer and self-stitched fabrics of the prior art the internal surface of the wear side fabric layer is dominated by floats of MD yarns. Where wear side fabric CD yarns interlace with wear side fabric MD yarns, such that the wear side CD yarns appear in the internal region between the paper side and wear side fabric layers, relatively prominent short weft knuckles are formed. The pressure of relatively stiff wear side MD yarns acting on the wear side CD yarns during the production of the fabric can cause so-called “knuckle spread,” whereby the wear side CD yarn knuckles are distorted and their width increased to form a relatively large area. The location of such yarn mass areas within the fabric inner region reduces the ability of water to flow through the fabric in such yarn mass areas such that fabric dewatering may be adversely affected.
A further common feature of the known self-stitched and other triple-layer designs is that they are relatively thick structures with a high amount of empty space distributed throughout their thickness. The relatively high “void volume” is typically associated with sheet rewetting on the paper machine such that the sheet solids content at transfer to the press section may be undesirably low. That is, the fibrous web formed on the papermaking fabric has an undesirably low fiber-to-weight ratio. This can result in reduced papermaking machine performance through a higher amount of sheet breaks occasioned by the wetter sheet, reduced running speed and higher drying costs downstream of initial web formation.
The prior art '303 patent, referred to earlier herein, discloses embodiments which utilised more, and finer warp yarns in the paperside layer as compared to the warp yarns in the wear side layer. The wear side warp yarns in the '303 patent occur less frequently and are of a thicker diameter than the paper side warp yarns. Accordingly, the embodiments shown in '303 would have a relatively high thickness and corresponding void volume. These issues have been addressed somewhat by more recent patents, including U.S. Pat. No. 5,826,627 (Seabrook, et al.); U.S. Pat. No. 5,967,195 (Ward), and U.S. Pat. No. 6,145,550 (Ward). The entire subject matter of all these latter-identified patents is incorporated herein, by reference. All of these latter patents disclose self-stitched triple-layer fabrics with equal numbers of warp yarns in their paper side and wear side fabric layers. Consequently, there is still adequate MD orientated load bearing material, but a reduction in fabric thickness and void volume as compared to the embodiments shown in '303 patent is expected.
More recently, thickness and void volume of self-stitched fabrics have been the subject of EP 1,273,698, the subject matter of which is fully incorporated by reference herein. This latter publication describes the provision of self-stitched structures with reduced thickness and void volume compared to the prior art. To achieve such structures high numbers of thin paper side yarns are required to allow the fabric of the '698 publication to perform adequately. Thus processing costs are higher for such fabrics, as compared to fabrics requiring a lesser number of relatively thicker fabrics. Furthermore, the lifetime of such fabrics may be limited because a wear side CD yarn of relatively small diameter is typically chosen to facilitate the recessing of the thin, load bearing, wearside MD yarns into the fabric and away from the wearing elements of the paper machine. By comparison with the prior art fabrics, the embodiments of '698 publication are relatively thin and consequently are more prone to instability than such prior art fabrics. Consequently, sheet basis weight profiles may be adversely affected by the use of fabrics made in accordance with the teachings of the '698 patent.
Although the aforementioned composite papermaking fabrics employing intrinsic interchanging binder yarn pairs have provided improved structures, applicant believes that there still is a need for additional, improved composite structures of the type employing intrinsic interchanging binder yarn pairs providing a high resistance to delamination, or separation, of the paper side and machine side layers, and also providing a composite fabric of relatively low thickness and related low void volume, to improve paper sheet dryness, and with suitable fabric stability to produce sheets with good basis weight profiles, and further providing a composite fabric with high levels of wearable material on the fabric wear side layer to perform without mechanical problems for the desired duration of use. It is to such structures that the present invention is directed.